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Revenue like this is usually paid for a service. Ultimately I thought the service was intermediation between savers in China, Japan and the Middle East (who want Treasuries) and dis-savers in the Anglo countries (who want to fund exotic credit card debt and mortgages). That remains the only service that looks large enough to justify that sort of revenue. (The real service having been finding suckers such as municipalities and insurance companies to hold the toxic waste such as CDO squared re-securitization paper.)
That said, given almost nobody knows how to make $22 billion per annum trading and jealousy is a common trait, conspiracy theories abound. The current conspiracy theory is that this money comes from front-running clients in the market with very rapid trading. The New York Times recently promoted this view.